Electric motors convert electrical energy into mechanical energy. In electric motors of normal construction, the basic parts, such as a rotor with a shaft fitted to rotate, a stationary stator, bearings and end shields, can be distinguished. The rotor is situated so as to be supported by the bearings. Generally a small air gap is left between the rotor and the stator.
The operation of multiphase alternating-current rotating machines, such as a multiphase synchronous and asynchronous motor, is based on a magnetic field circulating inside the machine. A multiphase stator winding is formed such that a sinusoidal voltage is fed into the phase windings, with the voltages fed into the windings being at a 360/m angle to each other in the phase shift, where m is the number of phases, the currents passing through the stator windings thus creating a magnetic field that circulates the air gap in the machine, said magnetic field interacting with the magnetic field of the rotor windings thereby making the rotor rotate. The magnetic field in the rotor winding of synchronous machines is typically formed from either a permanent magnet or with direct current fed into the excitation winding of the rotor. Magnetization of the rotor winding in asynchronous machines is generally implemented via the voltages and currents induced in the rotor winding caused by the magnetic flux of the stator current.
The aim is for the distribution of the magnetic flux density of the air gap to be as purely sinusoidal as possible. The rotating motion of the rotor is achieved by means of the fundamental sinusoidal wave of the magnetic flux density, but in practice the magnetic field affecting a motor also contains harmonic terms, i.e. harmonic components of the pure wave.
The harmonics of the magnetic flux density cause extra force components between the stator and the rotor. Furthermore, the magnitude of the torque fluctuates (torque ripple) and additional losses occur in the motor. If the frequency and form of fluctuation of the of the force caused by a magnetic field containing harmonics are close to the mechanical natural frequencies of the motor, a loud noise and vibration of the machine can occur as a result of the harmonics. Further, constrained vibration is possible. In constrained vibration, forces are exerted on a component that cause it to vibrate, although the frequency of the excitation is not the natural frequency of the component. Additionally, harmonics can lead to faulty operation of measuring and protective equipment, to overvoltages and to overload situations.
In three-phase electric motors only odd harmonic terms of the magnetic field occur. Prior-art solutions have attempted to minimize the effect of harmonics by changing the basic winding of the stator, with a fractional-pitch winding, with slot wedges and with dispersed placement of the magnets. In modern motors, however, vibration and noise caused by the force components occurring at 6 times and 12 times the frequencies with respect to the frequency of the motor current have been evident, said force components resulting especially from the 5th, 7th, 11th and 13th harmonic terms of the flux density.
Harmonic components occur in the air gap flux density in a rotating electrical machine owing to both discontinuity of the windings on the rims of the stator and rotor and from fluctuations in the permeance in the air gap. The stator winding is generally concentrated in slots and coil groups, in which case the magnetomotive force produced in the air gap is not sinusoidally distributed. Permeance fluctuation in the air gap is caused by, among other things, possible slotting of the stator and rotor, salient poles and magnetic saturation. The harmonics of the magnetic field of an electric motor can be divided into harmonics caused by the rotor and harmonics caused by the stator.
Torque ripple occurs in other rotating field machines also, but the following addresses in particular permanent magnet synchronous motors, which can be axial flux or radial flux machines. In an axial flux machine the magnetic flux of the air gap of the machine is situated mainly in the direction of the shaft of the machine. In a radial flux machine, on the other hand, the magnetic flux of the air gap of the machine passes mainly in the radial direction with respect to the shaft.
Reduction of torque ripple caused by the rotor of permanent magnet machines is addressed, for example, in patent application US2004/0070300. In the solution presented in this publication the magnetic field caused by the rotor magnets is made as purely sinusoidal as possible by making the rotor magnets pole shaped and by skewing their placement. Solutions for reducing torque ripple caused by the rotor are also presented in, for instance, publications U.S. Pat. No. 6,380,658 and U.S. Pat. No. 5,886,440. Prior-art solutions also include reducing torque ripple caused by the rotor by dispersed placement of the magnets.
The publication written by Y. Akiyama et al., “Slot Ripple of Induction Motor and FEM Simulation on Magnetic Noise”, Proceedings of the IEEE IAS 31st Annual Meeting, San Diego, USA, 1996, p. 644-651, addresses random placement of the slots of the rotor. The publication presents the reduction of the magnetic noise of induction motors by using non-equidistant distribution of the rotor slots. Three different types of rotor slotting principles (methods A, b and C) are presented. In methods A and B the rotor slots are situated completely randomly. The simulation result showed that the motor was very susceptible to saturation at the location of very thin teeth. In method C the slotting of the rotor is divided into quarters of the rim, and in each quarter the distance between slots is constant. Between adjacent quarters is a small displacement. Rotor A gives the best result in terms of interference components.
As previously stated, torque ripple is also caused by the stator, as a result of both harmonics caused by the discrete distribution of current in the circumferential direction of the stator and permeance fluctuation in the air gap caused by the stator slotting, for which the aforementioned publications do not offer a solution.
Prior-art solutions have attempted to reduce harmonics caused by distribution of the stator current with, among other things, a fractional-pitch winding or by using skewed slots. A fractional-pitch winding can eliminate slot harmonics of a certain order, but it cannot affect slotting harmonics. Skewed slots also distribute permeance on the rim more evenly, but using skewed slots complicates the process of manufacturing the motor and also reduces the torque available from the motor. It is known that using a magnetic slot wedge at the mouth of slots reduces permeance fluctuations caused by the slotting. By means of a slot wedge the permeance fluctuations can be made more even and the amplitude of certain harmonics reduced. For example, publication FI 112412 presents a method for manufacturing the winding of an electrical machine. In this method the winding coils are formed into their final shape before being placed in the slots. The winding coils are then placed so that they overlap, one coil being disposed at the base of the slot and the other coil placed on top of it. Additionally, in the method the slots are closed after placement of the winding coils with ferromagnetic slot wedges. By means of the slot wedges and by using fractional-pitch winding the harmonic terms can be damped to about one-quarter of the magnitude compared to a motor without slot wedges.
Publication. U.S. Pat. No. 6,285,104 presents a solution for reducing torque ripple wherein a different number of conductors can be placed in the stator slots such that the current vector fed sinusoidally into each slot is formed as similarly to the current vectors of the other slots as possible. In this method the width of the stator slot is determined by the number of conductors contained in the slot. The method also presents moving the rotor magnets in the direction of the circumference with respect to the stator. One drawback in the solution presented is, among other things, that it makes the process of manufacturing the stator and stator winding more difficult.
In prior-art solutions mechanical vibrations occurring in the motor are damped, as presented in e.g. publication WO 9826643. According to this publication a second voltage is fed into the current supply of the motor, the frequency of which is a certain multiple of the fundamental frequency. The frequency depends on the number of phases and on the number of stator slots per phase.
Based on publication FI 950145, it is a prior-art technique to manufacture the magnetic core (stator) of an axial motor as a cylindrically-shaped stack of plates in the following manner. A ribbon-like ferromagnetic plate is coiled into a cylindrical stack of plates either spirally or annularly. Before coiling into a roll, the exact positions of the stator slots on the plate are calculated and the slots are punched while the plate is in a straight plane with a special punching and slotting machine. The punching locations are not positioned equidistantly because the radius of the plate mass accumulating around the centre axis of the plate stack changes during the coiling. When the plate stack is fully coiled, the stator slots in the stack are located in the desired positions and are of the desired depth, and the walls of the slots are even.
The problem with this prior-art solution is that the vibration and noise caused by harmonics are not reduced in the best possible way with the prior-art methods. For example, the vibration caused by torque ripple in an electric motor in elevator usage can still be noticed as vibration and jerky motion of the elevator car. The noise caused by harmonics can also reduce passenger ride comfort.